Miniaturization of electric devices has been achieved for a long time, and development of lots of portable electric devices has been made. However, recently, as mobile communication is rapidly developed and a new paradigm called the ubiquitous network is introduced, the development of small and easy-to-carry electric devices is on the spotlight. Most electric devices such as MP3 players, digital cameras, mobile phones, PDAs, laptop computers, etc., are made compact and are being evolved in an easy-to-carry form. In addition, an attempt to incorporating various functions into a single device such as an MP3 player, a digital camera, etc., is made along with such miniaturization of electric devices. This attempt provides convenience of use and freedom of migration to a user, but stable supply of electric power is proposed as a new task.
Conventionally, a battery has been widely used as means for supply of power to electric devices. For example, primary batteries such as manganese batteries, alkali manganese batteries, zinc-air batteries and so forth, and secondary batteries such as nickel-cadmium (Ni—Cd) batteries, nickel-metal hydride (Ni—H) batteries, lithium ion batteries and so forth has been used as the power supply means. Among these, the zinc-air battery has a merit that it provides a relatively high voltage of 1.4V and has a high energy density and a large discharge capacity. Further, the zinc-air battery is considered to be a battery capable of substituting for a mercury battery that is restricted in use due to heavy metal content, since it exhibits a nearly uniform discharge characteristic until the discharge of the battery is completed.
Referring to FIG. 8, a conventional button-type zinc-air cell (battery) includes a membrane 14 as an anode, a zinc gel 12 as a cathode and a separator 16 interposed between the membrane and the zinc gel for insulating the membrane and the zinc gel. The membrane and the zinc gel are accommodated in a conductive anode can 20 and conductive cathode can 22, respectively, to form the zinc-air cell.
The membrane 14 is a permeable membrane containing water molecules, which is in contact with oxygen in the air to generate hydroxide ions (OH−). This reaction can be represented in the following chemical formula:O2+2H2O+4e−4OH−  Chemistry Figure 1
In the above reaction, electrons are supplied to the membrane 14 through the anode can 20. Carbon is typically used as a material for the membrane, but a suitable material may be used according to a required voltage or an applicable field.
As such, oxygen is needed for the reaction in the anode, hence the anode should have a path allowing for being in contact with air. Thus, the anode can 20 has air holes 28 formed at the bottom thereof. When the cell is not in use, the air holes 28 are hermetically sealed to suppress the reaction in the anode.
The hydroxide ions generated in the above chemical reaction are transferred to the zinc gel, which is the cathode, through the separator 16. The separator 16 has permeability for the hydroxide ions and has a function of preventing leakage of the zinc gel and insulating the zinc gel from the membrane.
The zinc gel mainly contains zinc powder and has a mixture of an additive and an electrolyte. Typically, potassium hydroxide (KOH) aqueous solution is used as the electrolyte. When the hydroxide ions are transferred to the inside of the zinc gel, the zinc powder reacts with the hydroxide ions to be oxidized. This reaction can be represented in the following chemical formula:Zn+2OH−Zn(OH)2+2e−Zn+2OH−ZnO+H2O+2e−  Chemistry Figure 2
By this reaction, electrons are generated from the cathode and then are transferred through the cathode can 22. A maximum voltage of 1.65V can be theoretically generated by such a chemical reactions.
The conventional zinc-air cell was mostly used in only a button-type and in such a button-type zinc-air cell the hermetical sealing thereof is performed using the bending of the can. A method of producing a conventional zinc-air cell is disclosed in Japanese Patent Laid-Open Publication No. 2002-373711.
The method of producing a conventional zinc-air cell will be described hereinafter with reference to FIG. 9. Referring to FIG. 9, the zinc-air cell includes a zinc gel 12 as a cathode, an anode membrane 14 as an anode, and a separator 16 interposed between the membrane and the zinc gel for insulating the membrane and the zinc gel. A cathode can 22 and an anode can 20 that are in contact with the zinc gel 12 and the anode membrane 14, respectively, capture the zinc gel and the anode member. Meanwhile, the anode can 20 has through-holes 28 formed at the bottom thereof so as for the anode membrane (14) to be in contact with air.
A gasket 26 is interposed between the cathode can 22 and the anode can 20 at a distal end of the can, and the anode can 20 and the gasket 26 are bent toward the cathode 22 to achieve the hermetical sealing of the cell.
Such a zinc-air cell has an advantageous property in terms of voltage, energy density, discharge capacity, discharge characteristic, etc. Nevertheless, the application of the zinc-air cell is limited to a special field such as a hearing aid, a camera and the like. Particularly, the zinc-air cell is sold only as a button-type cell and is not manufactured to conform to a cylindrical-type standard specification such as AAA, AA, etc. In order to commercialize a cylindrical zinc-air cell, it is required that the zinc-air cell should be fabricated so as to generate voltage and current suitable for an applicable field to which a cylindrical cell is applied as well as a method itself of producing a cylindrical zinc-air cell should be developed.
A problem associated with the cylindrical zinc-air cell fabricating method will be described hereinafter with reference to FIG. 10. FIG. 10 is a cross-sectional view illustrating a virtual cylindrical zinc-air cell. In FIG. 10, an identical reference numeral is used for the same element as that in FIG. 8. The zinc-air cell includes a zinc gel as a cathode, and hence leakage of zinc gel must be prevented. In the conventional button-type zinc-air cell as shown in FIG. 8, since the anode membrane 14 and the separator 16 are disposed at the lower portion of the zinc gel so that leakage of zinc gel can be prevented, the button-type zinc-air cell is manufactured easily. However, the cylindrical zinc-air cell as shown in FIG. 10 has a structure in which a separator 16 and an anode membrane 14 capture a zinc gel 12. In this case, the anode membrane 14 and the separator 16 have a junction 30 in formation of a cylindrical shape, and hence it is not easy to avoid leakage of the zinc gel.
Therefore, in order to fabricate such a cylindrical zinc-air cell, there is a need for a method of bonding a junction of the anode membrane 14 and the separator 16 while preventing leakage of the zinc gel.